MURPHY and SHOMURA: PRE-EXPLOITATION ABUNDANCE OF TUNAS 



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LATITUDE 



Figure 26. — Variation in yellowfin tuna catch with the 

 current system for the two 6-month periods, January- 

 June, a period with northeasterly or light and variable 

 winds, and July-December, a period of prevailing south- 

 easterly winds (in the equatorial central Pacific). 

 (Adapted from King and Hida, 1957,) 



of continuity of measurements of the fish and 

 the environment. 



To effect this type of analysis we must use 

 some property in the environment that is con- 

 sistently measured at each fishing station. At- 

 mospheric data, such as winds and water tem- 

 perature, are available. Winds are not suitable, 

 because they are not conservative and because 

 theory holds that there is a considerable lag be- 

 tween wind action and the response of the fish. 

 Water temperature is more promising, since it 

 is relatively conservative and is also a property 

 of the immediate environment of the fish.^ 



In effect the study becomes limited to an anal- 

 ysis of variation along separate meridians or 

 groups of adjacent meridians, because the water 

 on widely spaced meridians is in all likelihood 

 different with respect to origin and properties, 

 such as the temperature-phosphate ratios. Fur- 

 thermore, we have restricted the analyses to the 

 zone north of the equator (lat 1°-5°N), where 

 sampling was most intensive and where the the- 



^ It might be argued that surface temperature does 

 not represent any substantial body of water. Fortunate- 

 ly, in the zone north of the equator the surface temper- 

 ature is generally representative of the entire mixed 

 layer, providing the effect of diurnal heating is sup- 

 pressed. This has been done by using temperatures 

 measured at about 9 m below the surface. 



ory of circulation is most advanced (Cromwell, 

 1953). The immediate vicinity of the equator 

 is avoided on the basis of possible complications 

 arising when this site of upwelling is considered 

 and the unknown influence of the easterly flow- 

 ing undercurrent. 



Defining the geographical unit to be used in 

 the time variation analysis as a single longitude 

 or immediately adjacent longitudes between lat 

 1° and 5°N permits the development of a hypoth- 

 esis. Wind action causes upwelling at the equa- 

 tor and a northward movement of upwelled 

 water. This water, which might be termed 

 "new," is cold, high in phosphate, high in plank- 

 ton, and low in yellowfin tuna abundance. The 

 meridional sections indicate that after it moves 

 north, away from the possibility of further en- 

 richment, it warms; phosphate declines, pre- 

 sumably from use; zooplankton declines, pre- 

 sumably from grazing; and yellowfin tuna abun- 

 dance increases, presumably because the tunas 

 congregate to feed on the forage organisms re- 

 sponsible for the decline in zooplankton. Water 

 that has reached this final stage can be termed 

 "mature." 



This same reasoning can be applied to tem- 

 poral variation in water temperature in places 

 removed from the site of upwelling, i.e., the zone 

 between lat 1° and 5°N. In this zone, the pres- 

 ence of cooler water must mean that the wind- 

 driven northward movement was or is more vig- 

 orous and that the water there has been more 

 recently enriched and has been in the euphotic 

 zone for a shorter period of time. Consequently, 

 it should contain more phosphate, more plankton, 

 and fewer yellowfin tuna. Warmer water must 

 have been longer in the euphotic zone, perhaps 

 the result of a slowing of the wind circulation, 

 and should contain less phosphate, less plankton, 

 and more yellowfin tuna. 



Phosphate. — If the hypothesis outlined above 

 is correct, the concentration of phosphate in the 

 ocean water north of the equator should be less 

 in warmer water than in colder water, without 

 regard to the time of sampling or place of sam- 

 pling, providing consideration is restricted to the 

 zone between upwelling and the Countercurrent 



897 



